Mgix and Sixi line ratios in the sun

New theoretical emission line ratios for the Be-sequence ions Mgix and Sixi are presented. A comparison with observational data for two solar flares and an active region loop obtained with the Harvard EUV spectrometer and NRL XUV spectroheliograph aboard Skylab reveals that these plasmas are in ionization equilibrium at coronal temperatures. Unfortunately most of the density diagnostics are not particularly useful under solar plasma conditions, as they vary only slightly over the electron density range 10⁸–10¹³cm^–3. However the Sixi ratioI(³ P ^e ₂ -³ P ^o ₂)/I(³ P ^o ₁ –¹ S ^e ₀) is density sensitive in the range 10⁸ to 10¹⁰cm^–3, which is representative of electron densities found in solar active regions or small flares.     

E-region effective electron loss rates and electron temperatures under different solar flare conditions

Presuming the total life-time for recombination of molecular ions with electrons remains constant at each altitude under different solar flare conditions, one can estimate the mean dissociative recombination coefficient and the electron temperatures, with a knowledge of the quiet time recombination coefficient and the production rate profiles under these conditions. To estimate the electron temperatures, the temperature dependence of the form (300/T _e )^1/2 for the electron loss coefficient has been assumed. The computations show very high electron temperatures below about 130 km for the representative solar events considered.     

On the thermal structure of Uranus

Uranus has an effective temperature close to the solar equilibrium value and undoubtedly a thermal inversion of at least 140 K at a pressure of ～3 dyncm^?2. With the inversion and the thermal opacity provided by a HeH₂ mixture in a ratio close to solar abundance, acceptable agreement can be achieved with the available infrared observations. The cause of the inversion is, however, uncertain. The use of the HeH₂ opacity for Uranus is justified by the excellent agreement of the frequency variation of that opacity with the thermal spectrum of Jupiter.     

Physical processes determining the chromospheric temperature distribution

Calculations performed with several models of the solar chromosphere support Ulmschneider's conclusion that relatively short period acoustic waves heat the low chromosphere in the region just above the temperature minimum. However, these same short period waves (10 period P80 s) are not able to maintain chromospheric temperatures at heights where _5000Å(normal) < 10^-6. The calculations also show that an earlier conjecture stating that the H₂ population might influence the non-LTE chromospheric H^- population is probably not correct, due to lower values of the ratio n _e/n _H inferred from more recent observations. Finally, the calculations support Athay's contention that the Cayrel mechanism alone cannot produce the observed temperature rise, because the magnitude of the radiative cooling in the lines is too great.     

Electron densities derived from line intensity ratios: Beryllium isoelectronic sequence

A direct method for determining electron densities from emission line intensities of ions in the beryllium isoelectronic sequence is described and then applied to the analysis of extreme ultraviolet Ciii and Ov spectra from both quiet and active areas in the solar transition region. The results are consistent with a value of N _e T _e = 6 × 10¹⁴ cm^-3K for the quiet Sun at temperatures of 5 × 10⁴ to 3 × 10⁵K. Electron densities are approximately five times greater in active regions than in the quiet Sun.     

The EUV chromospheric network in the quiet Sun

Investigations on the structure and intensity of the chromospheric network from quiet solar regions have been carried out with EUV data obtained from the Harvard spectroheliometer on the Apollo Telescope Mount of Skylab. The distribution of intensities within supergranulation cell interiors follows a near normal function, where the standard deviation exceeds the value expected from the counting rate, which indicates fine-scale structure below the 5 arc sec resolution of the data. The intensities from the centers of supergranulation cells appear to be the same in both quiet regions and coronal holes, although the network is significantly different in the two types of regions. The average halfwidth of the network elements was measured as 10 arc sec, and was independent of the temperature of formation of the observing line for 3.8 < log T _e < 5.8. The contrast between the network and the centers of cells is greatest for lines with log T _e 5.2, where the network contributes approximately 75% of the intensity of quiet solar regions. The contrast and fractional intensity contributions decrease to higher and lower temperatures characteristic of the corona and chromosphere.     

Convection regions and coronas of sun and stars

Photospheric models were calculated for 90 stars with effective temperatures between 2500 K and 41600 K for five logg-values ranging from 1 to 5. Molecule formation was taken into account. In order to have an idea about possible instabilities in the different stellar layers some quantities, characteristic for convection and turbulence were calculated, such as the Rayleigh-, Reynolds-, Prandtl- and Péclet-numbers. It turned out that all the investigated stars contain unstable layers, including the hottest. Nevertheless, only stars with effective temperatures of 8300 K or less contain layers where the convective energy transport is important. For all stars the convective velocities were calculated and also the generated mechanical fluxes in the convection zones were tabulated.Under the hypothesis that this mechanical energy flux is responsible for the heating of the corona, coronal models were constructed for the Sun and for some stars with effective temperatures between 5000 K and 8320 K for logg-values of 4 or 5.For Main Sequence stars the largest fluxes are generated in F-stars; stars withT _eff=7130 K and logg=4 possess also the hottest and most dense coronas with a computed temperature of 3.7·10⁶ K and logN _e =10.5.The solar corona computed in this way, on the basis of a photospheric mechanical flux of 0.14·10⁸ erg cm^–2 sec^–1, has a temperature of 1.3·10⁶ K and logN _e =9.8. This density is apparently too high, but even when including in the computations all theoretical refinements proposed in the last few years by various authors it does not appear possible to obtain a solar coronal model with a smaller density.However, when taking into account the inhomogeneous structure of the chromosphere and by associating the calculated mechanical fluxes to the coarse mottles, and lower fluxes to the undisturbed regions we find a mean coronal temperature of 1.1·10⁶ K and a mean logN _e -values of 9. The computed velocity of the solar wind at a distance of 10⁴ km above the photosphere has a value between 7 and 11 km sec^–1. These latter values are in fair agreement with the observations.     

On the coronal lines in the chromosphere at the 1970 eclipse

Spectrographic observations of the flash spectrum were made by the Kwasan Observatory at the total solar eclipse on 7 March, 1970. The integrated intensities of Fexiv 5303, Fex 6374, and the continuum were measured on the spectrograms as a function of height above the Sun's limb. It was found that a large amount of emission in the coronal lines originates in the interspicular regions of the chromosphere. Analysis of the data yielded that the interspicular regions consist of coronal material of T _e = 1.6 × 10⁶–1.2 × 10⁶ and log N _e = 8.5–9.5, and that a decrease in T _e and an increase in N _e occur with decreasing height.     

High resolution EUV structure of the chromosphere-corona transition region above a sunspot

Ion emission line intensities between 1170 and 1700 Å allow one to determine the differential emission measure (DEM) and electron pressure of the plasma in the solar transition region (TR). These line intensities together with their Doppler shifts and line widths are measured simultaneously for the first time above a sunsport from data obtained with the NRL High Resolution Telescope and Spectrograph with 0.06 Å spectral and 1 spatial resolution.The Doppler shifts show both subsonic and supersonic flow in the same line of sight over the umbra. The temperature structure for 40 resolution elements in the sunspot umbra and penumbra is derived from the DEM and the observed electron pressures.Extrapolation of the emission measure curves supports the previous EUV and X-ray observations that coronal plasma above sunspots with T _e>10⁶ K is reduced while emission from TR plasma between 2×10⁵ and 10⁶ K is greatly enhanced relative to quiet or active regions. This enhancement shifts the minimum of the DEM to lower temperatures and increases the slope at 2×10⁵ K by a factor of two.New pressure diagnostics using the emission line intensity ratios of C iv to N iv are presented, and applied to the data.The energy balance in the TR for the sunspot umbra is dominated by radiative losses from the large amount of TR plasma.An estimate of the energy budget shows that an energy input is required to balance the radiative energy losses above the umbra. The observed divergence of the enthalpy flux for the umbral downflows can balance these radiative losses for T _e between 30000 and 200 000 K.A typical umbral model of T _e versus reduced mass column density is compared with one for chromospheric temperatures determined from the Ca H and K lines.Institute of Theoretical Astrophysics, University of Oslo, Norway.     

Theoretical Ne v emission line ratios compared to solar observations

The recent level population calculations for Ne v by Aggarwal are used to determine the theoretical emission line ratios R ₁ = I(2s2p ^{3 1}D^o - 2s²2p^{2 1}D^e)/I(2s2p^{3 3}D ₂ ⁰ - 2s²2p^{2 3}P ₁ ^e ) and R ₂ = I(2s2p ^{3 1}D^o-2s²2p^{2 1}D^e)/I(2s2p ^{3 3}D ₃ ⁰ -2s²2p^{2 3}P ₂ ^e ). A comparison of these with observational data for a solar flare and erupting prominence obtained with the NRL XUV spectrograph on board Skylab reveals that R ₁ and R ₂ are in their predicted high density limits. Although the ratios cannot be used as density diagnostics for values of n _e typical of the solar transition region, it is shown that they are temperature sensitive and hence may be employed to determine the electron temperatures of Ne v line emitting regions.     

Thallium in the solar atmosphere

Umbral spectra are shown to contain an absorption feature attributable to the Tl i transition 6p ² P°_3/2–7s ² S _1/2 at 5350 Å. Analysis of the umbral spectrum suggests a solar abundance in the 0.72< log N(Tl)T<1.10 on the standard scale log N(H) = 12.00. Unidentified blends limit the accuracy of the abundance determination.     

Si III electron temperature diagnostics for the solar transition region

R-matrix calculations of electron impact excitation rates for transitions in Si iii are used to derive the electron-density-sensitive emission line ratios R ₁ = I(1113.2 Å)/I(1206.3 Å), R ₂ = I(1298.9 Å)/I(1206.3 Å), and R ₃ = I(1296.7 Å)/I(1206.3 Å). A comparison of these with observational data for several solar features obtained with the Harvard S-055 spectrometer on board Skylab reveals that theory and experiment are compatible if the electron temperature of the Si iii emitting region of the solar atmosphere is log T _e = 4.5, but not if log T _e = 4.7. The implication of the choice of a lower temperature on the electron energy distribution function is also briefly discussed.     

The abundance of chlorine in the Sun

The Cli line 8375.943 (4s ⁴P_5/2 – 4p ⁴D7/2) is identified in the solar spectrum. This is the first identification of a chlorine line in solar spectrum. The measured equivalent width (W = 0.8 mÅ) corresponds to an abundance log N(Cl) = 5.65 on the scale log N(H) = 12.00.     

Study of the June 30, 1973 trans-polar coronal hole

Radially and tangentially polarized pictures of the solar corona obtained near 4500 Å during the 30 June, 1973 solar total eclipse have been used to derive a model of a trans-polar coronal hole. The hole is identified by using OSO-7 EUV spectroheliograms. The line of sight coincides with the privileged plan of the hole over the N-polar region. A new method of absolute calibration is used. The Saito (1970) method is applied to determine the electron densities. Extrapolated values of densities down to the surface are lower than have ever been observed although derived hydrostatic temperatures are certainly not: N _e × 10⁷ cm^–3 and T = 2 × 10⁶ K. The morphological peculiarities of polar regions are considered.On leave from Institut d'Astrophysique, CNRS, Paris as NRC Research Associate.     

Evidence for an Ionized Rotating Structure Around RW Aur

Profiles of the UV semiforbidden lines of C III and Si III of RW Aur have been obtained with the HST/STIS. The C III profile shows two high velocity components at v = ± 170 km s^?1 and a central one. The Si III profile is very broad (FWHM = 293 km s^?1) and the high velocity components are unresolved. It is shown that the high velocity components are most probably produced in a rotating belt alike the detected in other sources of bipolar outflows. A radius between 2.7 R _* and the corotation radius (6.1 R _*) is derived and a log T _e (K) ?4.7 and log n _e(cm ^?3) = 11.6 are estimated. The belt is clumpy and the most likely source of heating is local X-rays radiation, probably associated with the release of magnetic energy.     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Mg vii and Si ix line ratios in the sun

Recent R-matrix calculations of electron excitation rates for Mg vii and Si ix are used to determine the theoretical density sensitive emission line ratios R ₁= I(2s2p ^{3 1} D ⁰ - 2s ² 2p ^{2 1} D ^e )/I(2s2p ^{3 3} S ⁰ - 2s ² 2p ^{2 3} P ₂ ^e ) and R ₂= I(2s2p ^{3 1} P ⁰ - 2s ² 2p ^{2 1} D ^e )/I(2s2p ^{3 3} S ⁰ - 2s ² 2p ^{2 3} P ₂ ^e ). These are found to be quite similar to the earlier results of Mason and Bhatia. Electron densities derived using observed R ₁ and R ₂ ratios from Skylab NRL XUV spectra of solar flares and active regions are in good agreement, and compare favourably with those deduced from ions formed at similar electron temperatures to Mg vii and Si ix.     

A hydrocode equation of state for SiO2

Abstract— The thermodynamic properties of SiO₂ are approximated over a range of pressures and temperatures important under the extreme conditions achieved in impacts at typical solar system velocities from 5 to about 70 km/s. The liquid/vapor phase curve and critical point of SiO₂ are computed using the equation of state (EOS) program ANEOS. To achieve this goal, two shortcomings of ANEOS are corrected. ANEOS, originally developed at Sandia National Laboratories to describe metals, treats the vapor phase as a monatomic mixture of atoms, rather than molecular clusters. It also assumes a Morse potential for the expanded solid state. Neither of these assumptions is accurate for geologic materials, such as SiO₂, that contain molecular clusters in the vapor phase and are better described by a Mie‐type potential in the solid state. Using the updates described here, an EOS adequate for numerical hydrocode computations is constructed that agrees well with shock data at pressures up to at least 600 GPa and temperatures up to 50,000 K. This EOS also gives a good representation of the liquid/vapor transition at much lower pressures and temperatures. The estimated critical point parameters for SiO₂ are P_c = 0.19 GPa, T_c = 5400K, ρ_c = 550 kg/m³.     

A quantitative analysis of the surge of March 19, 1989

A large surge event appearing in AR 5395 was observed at the Yunnan Observatory on March 19, 1989. H spectral profiles of the event are interpreted by using a two-cloud model and the contours of three parameters: excitation temperature, T _exc, microturbulent velocity, V _t ,and column density of hydrogen atoms at the second level, N _0,2, are obtained, respectively. The question about the unique feature of the solution obtained by the fitting method is also discussed. The results show that the surge is composed of some conglomerated materials with higher temperature and density; the mass ejection is probably intermittent. Neither T _exc nor N _0,2 vary with the height over the solar limb but decrease from the center to the periphery of the surge. V _t varies from 10 to 30 km s^-1 and decreases with height. Some other important parameters, such as electron density, n _e ,and electron pressure, P _e ,etc., have also been estimated. In the surge, with N _0,2 about 2.0 × 10¹² cm^-2 and T _exc about 8500 K on average, we obtained n _e = 1.80 × 10¹⁰ cm^-3 and P _e = 0.023 dyn cm^-2. The energy variations of the surge during the ascending phase are estimated.     

Improved theoretical line ratios for Ciii in the Sun

The recent twelve-state R-matrix calculations of electron excitation rates in Ciii by Berrington are used to derive level populations applicable to the solar transition region. Line ratios R = I(2p ^{2 3} P ^e - 2s2p ³ P ^°)/I(2s2p ¹ P ^° - 2s ^{2 1} S ^e ) and R ₂=I(2p ^{2 1} S ^e - 2s2p ¹ P ^°)/I(2p ^{2 3} P ^e - 2s2p ³ P ^°) deduced from these data in conjunction with the relevent transition probabilities are found to be in much better agreement with the observed quiet Sun values than those determined from the level population calculations of Keenan et al.